1. Field of the Invention
The present invention relates to threaded connectors used to secure the ends of tubular bodies together. More particularly, the present invention relates to threaded connections provided with seals to protect the threads in pipe connections from exposure to corrosive fluids.
2. Setting of the Invention
Pipelines that extend through saltwater bodies are employed for multiple purposes, often associated with the drilling of oil and gas wells and the production and distribution of the oil and gas produced by the wells. When the pipelines are not buried or otherwise solidly anchored within the saltwater body, they are subjected to dynamic loadings that can eventually cause the pipelines to rupture or otherwise fail. The stresses induced by dynamic loading in the connections of the individual tubular bodies forming the pipeline are of particular concern.
Jointed pipelines that are secured together by the threaded engagement of pin and box connectors at the ends of individual pipe sections are employed in marine riser pipes as well as submerged pipelines and other bottom-to-surface supply lines. The marine riser pipelines are typically constructed of tubular pipe sections that are secured together at their ends by special connectors designed to withstand the destructive effects of the dynamic loading acting on the riser. The dynamic loading of the riser is caused by changing sea currents, changes in surface exerted tension resulting from wind and wave action against a surface support of the riser, and other factors.
The effects of dynamic loading on the connections used in marine applications may be offset, in part, by the use of connectors that have the strength required to withstand the forces imparted by the environment. Conventional riser connections typically employ heavy tool joint type connections that are welded onto the end of the pipe. These connections are very resistant to fatigue damage and typically employ a metal-to-metal torque shoulder as an external seal, similar to the design employed in a conventional drill pipe tool joint. Connections of this type have a very low stress concentration factor (SCF) and provide an environmental seal that prevents the seawater from contacting the threads in the connection.
The tool joint design is expensive to machine and requires the added step and expense of welding the connector to the pipe. The welding procedure also introduces a potential point of fatigue failure for the dynamically loaded connector. Moreover, as compared with a conventional threaded and coupled connection, the tool joint type connection is extremely heavy, requiring additional surface support.
Conventional threaded and coupled connections have been successfully employed as the outer riser in a marine installation for short time periods. Such connections cost less than ⅙ the cost of tool joint type connectors. The long-term reliability of these conventional connections in a saltwater environment under dynamic loading conditions, however, is not reasonably predictable. The problem stems from the fact that, when used as risers, the threads of conventional threaded and coupled connectors are exposed to salt water that can accelerate corrosion of the threaded area. The dynamic loading of the riser constantly flexes the connections laterally and imposes cyclical tension and compression stresses. The unengaged pin threads that have been machined into the pipe are exposed to the salt water. These exposed threads concentrate the stresses exerted on the pipe. Adding the effects of corrosion to the cyclical stress concentration unreasonably exposes the connection to a fatigue induced failure that will generally occur in the area of the last full. thread formed on the bore of the pin member of the connection.
The strength of a standard threaded and coupled connection is optimized by forming the pin threads such that the thread roots xe2x80x9crun outxe2x80x9d or continue to decrease in depth on the external surface of the pipe until they disappear at a point referred to as the xe2x80x9clast scratch.xe2x80x9d The final turns of the threads do not make a full depth cut into the pipe body and are not normally intended to be engaged and covered by threads in the mating coupling. From the point of the last full threaded engagement with the box threads to the last scratch of the pin threads, the pin threads in a conventional threaded and coupled connection are exposed to the surrounding environment. The corrosive effects of saltwater in this area of exposure can accelerate the failure of a connector subjected to cyclical, dynamically induced forces.
From the foregoing, it will be appreciated that a general object of the present invention is to prevent contact of the threads of a dynamically loaded connector with the corrosive fluids surrounding the connector.
A primary object of the present invention is to protect the exposed external pin threads in conventionally threaded connections of dynamically loaded pipe strings disposed in a body of saltwater.
Yet another object of the present invention is to provide a thread configuration on a non-upset pin that is sealed away from corrosive fluids to protect the pin from fatigue damage induced by cyclical, dynamic loading of the pin.
An important object of the present invention is to protect a conventionally threaded and coupled connection from the effects of saltwater exposure to the pin to prevent fatigue-induced damage when the connection is subjected to dynamic loading.
A related object of the present invention is to provide a marine riser pipe that weighs substantially less than a marine riser constructed with conventional tool joint type connections.
Yet another object of the present invention is to provide a marine pipeline that can withstand dynamic loading in a corrosive environment without the need for heavy, tool joint type connectors.
An object of the present invention is to provide a threaded pin connector that may be subjected to dynamic loading in a marine environment wherein the pin to the thread configuration redirects the imposed loads away from the last thread scratch area to minimize fatigue damage in the pin connection.
An object of the present invention is to protect the normally exposed pin threads in a pin and box connection from contact with corrosive fluid by providing an external metal-to-metal seal between the pin and box of the connection.
Yet another object to the present invention is to protect the normally exposed pin threads in a pin and box connection from contact with corrosive fluids by providing an external elastomeric seal between the pin and box of the connection.
It is also an object of the present invention to provide an external seal for protecting the normally exposed threads on a pin and box connection with a compression ring that actuates a protective seal between the pin and box without inducing significant mechanical stress in the connection.
Another object of the present invention is to provide a compression ring that encircles a pin and engages auxiliary threads on a box to advance the ring toward the box and compress an elastomeric seal between the box and pin whereby the normally exposed pin threads in the connection are isolated from contact with corrosive fluids.
An object of the present invention is to provide a compression ring that encircles the pin of a pin and box connector and is threadedly secured to threads formed on the external surface of the box whereby rotation of the compression ring compresses an annular, elastomeric seal between the pin and box to protect the normally exposed pin threads from contact with salt water. A related object of the present invention is to provide a compression ring that mates with threads formed on the internal surface of the coupling to compress the elastomeric seal ring.
The normally exposed pin runout threads on fully engaged, threaded and coupled connections are sealed from saltwater to prevent corrosion of the threaded area that can accelerate fatigue-induced failure in dynamically loaded pipelines. The seals may be employed with conventional threaded and coupled connectors permitting the fabrication of pipelines that are inexpensive and lightweight as compared with pipelines constructed with conventional tool joint type connectors.
The protective seal designs of the present invention permit the use of connections having runout threads that exhibit superior load-bearing characteristics in dynamically stressed applications.
An important feature of the present invention is that the pin connectors may be fabricated on the ends of non-upset pipe eliminating the expense and weight of conventional marine connectors.
Standard marine riser connections frequently include special groove designs and other connection configurations that redirect forces away from the areas of the pin most susceptible to fatigue induced failures resulting from cyclical, dynamic loading. Because they are protected from the effects of corrosive fluids surrounding the riser connections, the connectors of the present invention may employ the inherent force redirection effects of an inexpensive, conventional pin runout thread to achieve similar results.
The external seal portion of the present invention may be provided by metal-to-metal engagement between the pin and box or by an elastomeric annular seal compressed between the pin and box. The metal-to-metal external seal may be provided by engagement of the face of the box with a shoulder formed on the pin end of a non-upset pipe. The shoulder may be provided by an enlargement of the pin outside diameter. Where an annular, elastomeric seal is provided, the seal may be carried on the outer surface of the pin body or may be disposed in the box of the connector.
One form of the invention employs a compression ring threaded to the box to effect an external seal preventing salt water from contacting the normally exposed pin threads in a conventional threaded and coupled connection. The compression ring actuates an annular, elastomeric seal ring that isolates the normally exposed pin threads from saltwater contact. The compression ring cooperates with a second seal that protects the seal ring threads from saltwater exposure. The second seal may be either a metal-to-metal seal or an elastomeric seal. The seal ring provides the desired sealing of the exposed pin threads without imposing any structural stresses in the connection.
The foregoing objects and features of the present invention, as well as others, will be more fully understood and better appreciated by reference to the following drawings, written description and claims.